Here's why:

* Atoms are normally neutral: They have an equal number of protons (positively charged) and electrons (negatively charged).

* Gaining electrons creates an imbalance: When an atom gains electrons, it now has more negative charges than positive charges.

* Negative ions are formed: This excess of negative charge results in the atom becoming a negative ion.

Let me know if you'd like more details about ions or the process of gaining electrons!